When considering a person's vision, it is desired to describe that vision over the entire field of vision, both when looking straight ahead and when the eye gazes at different points in space. To do this, it is necessary to take account of the movement of the person's eyes, when said person gazes at different points of the field. The movement of the eye is commonly taken to be rotation about a particular point known as the Center of Rotation of the Eye (CRE).
The optical characteristics of an eye and lens system depend on the position of the CRE relative to the eyeglasses and it is normally considered that the CRE is placed on the primary gaze direction. Normally, using simple photography, it is possible to measure the distance between the rear face of the lens and the front face of the cornea along the primary gaze direction. The CRE is then positioned at a parameterized distance that may be equal to 15 millimeters (mm) behind the front face of the cornea.
More generally, the position of the CRE is measured relative to a point on the rear face or on the front face of the eyeglass lens, or a particular point on the person's face, relative to which the lens can be put in a known positioned.
These values normally used to determine the CRE are values that are theoretical, arbitrary, and standard.
Unfortunately, in practice, the position and the dimensions of the eye differ depending on the person, and, as a result, positioning the CRE by using such a theoretical value is approximate. More generally, the movements of the eye and thus the position of the CRE can be characterized by a function that is dependent on the gaze direction.